Abstract

A whitefly resistance crisis in Arizona in 1995
prompted the development of a resistance management
strategy in 1996 that recommended maximal once per season
use of two insect growth regulators, pyriproxyfen (Knack&reg)
and buprofezin (Applaud&reg),
and limited and delayed use of synergized pyrethroid
insecticides in cotton. Statewide monitoring of whitefly
resistance has shown that implementation of this strategy
has substantially reduced whitefly resistance to the
synergized pyrethroids and has also resulted in increased
susceptibility to key non-pyrethroid insecticides. Having
benefited from two years of success with this strategy,
the Arizona cotton industry now faces the question of
whether it can be sustained as pyriproxyfen and
buprofezin gain additional registrations for use against
whiteflies in vegetables, melons and glasshouse crops.

Introduction

Since the late 1980's, whiteflies have caused
severe damage to cotton, vegetable, and melon crops grown
in the irrigated deserts of the southwestern U.S. (Byrne
et al. 1990). In cotton, whitefly densities exceeding
5-10 adults per leaf can result in severe reductions in
lint quality due to stickiness caused by excreta
(honeydew) and subsequent development of sooty mold fungi
(Diehl and Ellsworth 1995). Intensive reliance on a
limited array of insecticides to combat whitefly
populations resulted in an insecticide resistance crisis
in Arizona cotton in 1995. This crisis prompted the
formulation and implementation of a successful whitefly
resistance management program (Dennehy and Williams
1997). In this paper we provide data from statewide
monitoring of whitefly resistance in order to assess the
success of the program at the end of its second year.

Materials and Methods

Monitoring of Whitefly Resistance to Insecticides

Whitefly were collected in 1997 from 12 locations
throughout the major cotton production areas of Arizona.
Adult whiteflies were vacuum-collected directly from
cotton foliage into plastic vials using a Makita&reg
cordless vacuum (4071D). Samples were transported in ice
chests directly to EARML where they were released into
cages containing young cotton plants, Gossypium hirsutum
L. (DPL-50). Adult whitefly were maintained in these
cages until they were placed in bioassays (<7 days).
Susceptibility of each population to the following
insecticide treatments was estimated: 1) Curacron&reg
(profenofos) + Lorsban&reg (chlorpyrifos) (1:1
ratio); 2) Curacron + Vydate&reg (oxamyl) (1:1
ratio); 3) Gowan Thiodan&reg (endosulfan); 4) a
fixed concentration of 1,000 µg/ml of active ingredient
Orthene&reg (acephate) combined with varying
concentrations of Danitol&reg (fenpropathrin);
5) Ovasyn&reg (amitraz); and 6) Ovasyn + Gowan
Thiodan (1:1 ratio).

A derivative of the Rothamsted leaf disk bioassay
method (Rowland et al. 1990) was used. Leaf punches 2.5
cm in diameter were taken from cotton plants (DPL-50) 18
- 26 days old. The leaf disks were dipped for 10 s in
formulated insecticide diluted in water. After drying,
the disks were placed individually on a base of agar
(1.3%) within 20 ml glass scintillation vials. Within 2 h
of dipping, 20-30 adult whiteflies were aspirated into
each vial. Assays were then held in an incubator at 27 &deg C
for 48 h, after which they were scored using a binocular
dissecting scope. Vials were tapped on the counter 10
times after which whiteflies not exhibiting repetitive
movement of more than one appendage were scored as dead.
At least five different concentrations and six bioassay
replications per concentration were evaluated for each
insecticide tested.

Results and Discussion

Danitol+Orthene Mixture

Table 1 presents
1997 results of whitefly susceptibility to mixtures of
Danitol+Orthene. Contrasts of 1995, 1996, and 1997
results from statewide collections show significant
increases in overall susceptibility to Danitol+Orthene
each of the past two years (Figure 1). Severe
resistance to the synergized pyrethroids in 1995 was
correlated with substantial (<80%) mortality (Figure 1) in bioassays of
100 µg/ml fenpropathrin (+ 1000 µg/ml acephate).

Based on mortality observed in bioassays of 10 µg/ml
fenpropathrin+1000 µg/ml acephate (Table 1), the populations
with lowest susceptibility to this insecticide mixture
were observed in Somerton and Central Arizona locations,
Buckeye, Coolidge, and Maricopa. Producers in these areas
should be especially careful to limit pyrethroid use to
two applications per season to avoid driving resistance
levels any higher. Sivasupramaniam et al. (1997) found
that resistance to Danitol+Orthene conferred
cross-resistance to all of the pyrethroids evaluated for
controlling whiteflies in Arizona. Our strategy continues
to emphasize holding the synergized pyrethroids in
reserve to be used as a last resort, should they be
needed late in the season when the crop is at greatest
risk of being contaminated by honeydew.

Our 1996 and 1997 data support the conclusion that the
Arizona Whitefly Resistance Management Program has
yielded benefits in terms of reductions in resistance to
synergized pyrethroids.

Thiodan

Table 2 presents
1997 results of whitefly susceptibility to Thiodan.
Contrasts of 1995, 1996, and 1997 results from statewide
collections show small increases in overall
susceptibility to Thiodan each of the past two years (Figure 2). Populations
with the lowest susceptibility to Thiodan originated from
Buckeye and Mohave Valley in 1997, based on mortality in
bioassays of 10 µg/ml (Table
2). The most susceptible populations were found in
Safford, Paloma and Coolidge.

Ovasyn

Table 3 presents
1997 results of whitefly susceptibility to Ovasyn.
Contrasts of 1995, 1996 and 1997 results from statewide
collections revealed a small increase in susceptibility
from 1995 to 1996 and a decrease in susceptibility to
Ovasyn in 1997 (Figure 3).
Ovasyn is an important component of the whitefly control
program in Arizona. As such, care should be taken to this
insecticide in rotations of products that will avoid its
overuse.

Thiodan+Ovasyn

There was no change in overall susceptibility to the
mixture of Thiodan+Ovasyn from 1996 to 1997 (Table 4). Buckeye, Marana,
and Maricopa populations were least susceptible to this
mixture and Yuma, Coolidge and Parker populations were
the most susceptible.

Curacron+Lorsban

Arizona whitefly populations differed significantly in
susceptibility to Curacron+Lorsban in 1997 (Table 5). In bioassays of
10 µg/ml, mortality was as high as 94-109% for the
Somerton and Yuma populations or as low as 13% for
Coolidge and Maricopa populations. However, based on
statewide means and medians, there was no significant
change in susceptibility to Curacron+Lorsban from 1996 to
1997.

Curacron+Vydate

Arizona whitefly populations were slightly more
susceptible to mixtures of Curacron+Vydate in 1997 than
1996 (Table 6).
Additionally, populations evaluated in 1997 differed
significantly in susceptibility to this mixture. The most
susceptible populations originated from Paloma and Yuma.
Populations least susceptible to Curacron+Vydate were
found at Marana and Safford.

Acknowledgments

We thank Cotton Incorporated, the Arizona Cotton
Growers Association and the University of Arizona for
financial support for this project. We thank P. Else and
the staff of the UA Campus Agricultural Center for
assistance with maintaining the EARML laboratories and
greenhouses.

This is a part of publication
AZ1006: "Cotton: A College
of Agriculture Report," 1998, College of
Agriculture, The University of Arizona, Tucson, Arizona,
85721. Any products, services, or organizations that are
mentioned, shown, or indirectly implied in this
publication do not imply endorsement by The University of
Arizona. The University is an Equal
Opportunity/Affirmative Action Employer.
This document located at
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